It is known how to actuate locking differentials by monitoring differences of speed rotation (similar to anti-blocking systems). Thus, a catalog of the SIGE Company, Vimercate (Italy) shows an "electronic locking differential" whose operating principle utilizes two speed-of-rotation sensors on the two wheels of shaft stubs, respectively, whose signals are compared electronically and then converted to an engagement signal for an electronically operable locking clutch. This type of construction is particularly unsatisfactory for vehicles with additionally clutchable drive axles for the following diverse reasons:
(a) The first reason is that the shifting principle exemplified by the SIGE catalog causes actuation of the lock immediately when a predetermined difference of rotational speed is reached, regardless of the cause. The synchronous rotation of the wheels thus attained leads again to a release of the lock. Therefore, a reactivation can occur immediately and it can, at an appropriate configuration of the terrain, come to a continuous actuation and deactuation which severely stresses the locking means. One attempts to counter this problem by means of an actuation delay. However, the problem will still divert a driver's attention.
(b) One needs, in addition, to two sensing positions, a relatively sensitive and complex electronic conversion means because an engagement point for an electrical coupling operated with switching current cannot be preset with mechanical exactitude.
(c) Furthermore, the prevention of accidental activation and deactivation of the lock during driving conditions when it is not desired (e.g. release of lock during turning of a tractor on strongly inclined, irregular terrain) based on comparison of speed of rotation is not attainable with absolute certainty. The reason for such uncertainty may be traced to the many influences of load distribution or terrain properties which are never exactly determinable in advance upon the release of the engagement actuation.
(d) Finally, the installation of two electronic measuring sensors to be attuned to each other on the two drive shafts increases or multiplies the problems of assembly, protection from damage, and inspection.
It is an objective of the present invention to advance the state of the art in such manner than an engagement means is created less complex and less prone to malfunction in an actuation range exactly predeterminable, easily installed, where the driver can clearly foresee the occurrence of the engagement point, and steer accordingly.